A very long time ago, 4000 years or more, China was being plagued by a great flood. People offered sacrifices to the river Lo, in order to appease it, but to no avail. The river’s only response was a turtle that emerged every time an offering was made. Then one day a little boy noticed some strange markings on the turtle’s back. They were representations of the numbers 1 through 9 arranged in a square. The four even numbers were in the corners of the square and the five odd ones formed the central cross:
Markings on the turtle’s back, above, representing the numbers shown below. [Images from Wikimedia Commons]